Relatively large and sophisticated web sites commonly implement some form of personalization system to provide personalized content, such as personalized item recommendations, to their users. To illustrate, some personalization systems can monitor and record one or more types of item-related user activity such as item purchases, item viewing events, and/or item rentals, and analyze the collected data to detect and quantify associations between particular items. When a user accesses a particular item, such as a particular product in a catalog of an e-commerce site, or an article on a news site, an appropriate message may be displayed notifying the user of related items (e.g., “people who bought this item also bought . . . ,” or “people who viewed this item also viewed . . . ”). The personalization system may also generate personalized item recommendations that are based on a target user's purchase history, item viewing history, item ratings, and/or some other type of user data.